Surely you could break the journey at darlo as it's an overnight stop?
Absolutely.
The question is whether, "by the book", the customer is compelled to arrange overnight accommodation and continue by rail the next day, or are free to travel by bus/taxi.
So, let's examine Condition 16:
NRCoC said:
you will be treated as breaking your journey if you leave a Train Companys or Rail Service Companys stations after you start your journey other than to:
(i) join a train at another station,or
(ii) stay in overnight accommodation when you cannot reasonably complete your journey within one day, or
(iii) follow any instructions given by a member of a Train Companys or Rail Service Companys staff
So if you don't stay overnight 16(ii) does not apply, so you could be in trouble if anyone found out you got a bus. In theory.
One way to avoid this would be to rely on 16(iii), e.g. the conversation would probably go like this:
Passenger: I think I will miss the last train from Darlington to Newton Aycliffe, would it be OK if I caught a bus?
Guard: Yes, that would be fine.
The Guard would probably think you're bonkers for asking, but of course we can't suggest anyone breaks the rules, so my advice has to be to ask.
(Or you could ask TPE or Northern on Twitter!)